Method and structure for finfet cmos

ABSTRACT

According to an embodiment, the invention provides an nFET/pFET pair of finFETs formed on a gate stack. At least one fin extends into a source drain region of each of the FET pair and a carbon doped silicon (Si:C) layer is formed on each such fin. Another aspect of the invention is a process flow to enable dual in-situ doped epitaxy to fill the nFET and pFET source drain with different epi materials while avoiding a ridge in the hard cap on the gate between the pair of finFETS. The gate spacer in both of the pair can be the same thickness. The extension region of both of the pair of finFETs can be activated by a single anneal.

FIELD

The subject matter disclosed herein relates to semiconductor structures.More specifically, the subject matter disclosed herein relates toforming complimentary field effect transistors of the fin type.

BACKGROUND

A finFET refers to a fin-type field effect transistor. Morespecifically, a finFET includes at least a “fin” of semiconductormaterial formed on a substrate such that the fin sidewall planes areorthogonal to the plane of the substrate, a gate electrode disposed onthe substrate and extending over and generally perpendicular to the atleast one fin, and a source drain region one either side of the gateelectrode. The at least one fins typically extend from under the gateinto both source drain regions.

The performance of a finFET and other structures based onsilicon-on-insulator (SOI) or extremely thin silicon-on-insulator(ETSOI) semiconductor substrates can be improved by forming thesource/drain (S/D) regions by in-situ doped epitaxy processing ratherthan by implant processes.

However, to impart compressive strain to the pFET channel and tensilestrain to the nFET channel, such as by forming in-situ boron doped(ISBD) SiGe for the pFET S/D and in-situ phosphorus doped (ISPD) Si forthe nFET S/D requires separate epi steps and that presents a number ofchallenges. One current flow scheme to form a complimentary pair offinFETs, meaning an n-type and p-type pair of finFETs, is illustrated bythe path of FIG. 1 which passes through odd numbered steps. The methodis applied to an initial structure that includes fins defined on asubstrate (104) and a gate stack is formed across the fins (106). Aspacer material such as silicon nitride can be deposited as a conformallayer over the gate and fins. A photoresist layer can be patterned (109)to form a soft mask cover over the nFET region and expose the pFETregion. (Although this example forms the pFET first, the order can beswitched to form the nFET first.) A first directional etch (111), suchas nitride RIE, to remove the exposed spacer layer (of 108) from thepFET S/D region exposes the pFET fins and forms the pFET gate spacer.After stripping (113) the soft mask, the pFET S/D can be formed (115) byepitaxial ISBD SiGe growth from the exposed pFET fins. The pFET can thenbe covered by depositing (116) a hard mask material, such as a secondsilicon nitride layer. A second photoresist layer can be patterned (118)to form a second soft mask covering the pFET region and exposing thenFET region which at this point includes the layer of step 108 and thelayer of step 116. A second etch (119), such as nitride RIE, can removeboth layers from the nFET S/D region to expose the nFET fins and formsthe nFET gate spacer. After stripping (121) the second soft mask, thenFET S/D can be formed (124) by epitaxial ISPD Si growth from theexposed nFET fins.

However, the dual in-situ doped epitaxy flow described above leads toseveral problems. For example, the gate spacer in the second to beformed FET is formed from two hard mask layers which may result in thatsecond-formed gate spacer being thicker than the gate spacer of thefirst formed FET (which is formed from just one hard mask layer).Another problem is that the drive-in anneal to activate the extensionrequires higher temperature for the pFET than for the nFET. Extensionrefers to the semiconductor region between source/drain and channelregion. In the finFET structure, the extension can include the portionof a fin that is under the gate spacer.

Accordingly, the first formed FET must undergo both anneals whichbroadens the dopant front at the extension junction which is deleteriousto short-channel control. Yet another problem is that the overlay of thefirst (109) and second (118) lithography can leave a double thicknessridge of hard mask (e.g., spacer) material over the gate at thetransition between an adjacent nFET and pFET pair.

There is a need for a CMOS finFET integration scheme that overcomes theabove-noted deficiencies.

BRIEF SUMMARY

Various embodiments disclosed include methods of forming semiconductorstructures. In one embodiment, a method includes:

According to a first embodiment, the invention provides a structure thatincludes a gate stack disposed over a plurality of fins on a integratedcircuit substrate. On a first portion of the gate stack is an nFET thatincludes a first subset of said plurality, a source drain region on bothsides of said first portion, and at least one fin of the first subsetextends laterally from said first portion into said nFET source drainregion. On a second portion of the gate stack and adjacent to the nFETis a pFET that includes a second subset of the plurality of fins, asource drain region on both sides of said second portion, and at leastone fin of said second subset extends laterally from said second portioninto said pFET source drain region. A carbon doped silicon (Si:C) layeris formed on the at least one fin in the pFET S/D and also on the atleast one fin in the nFET S/D. The nFET S/D can be filled with amaterial to impose tensile strain such as in-situ phosphorous dopedsilicon and the pFET S/D can be filled with a material to imposecompressive strain such as in-situ boron doped SiGe. The gate spacer ofthe nFET can be the same material and same shape and thickness as thegate spacer of the pFET. The Si:C layer can have a carbon contentbetween 0.2 to 3.0% and the Si:C layer can be between 1 nm and 3 nmthick.

Another embodiment of the present invention is a structure comprising ann-type and a p-type pair of finFETs in an integrated circuit whereineach finFET of said pair includes a gate electrode disposed over a setof fins, the structure further comprising a first gate spacer in saidn-type finFET and a second gate spacer in said p-type finFET, where thethickness at the base of said first gate spacer is the same as thethickness at the base of said second gate spacer. The pair of finFETscan be formed adjacent to each other on the same continuous gate stack.The maximum thickness of a hard cap on the gate between such adjacentpair of finFETs can be the same as (i.e., not greater than) the maximumthickness of a hard cap on the gate of either such adjacent pair offinFETs.

According to a third embodiment, the present invention provides a methodto form a complimentary pair of finFETs. The method includes (a) forminga spacer on a gate stack disposed over a plurality of fins, wherein suchgate stack extends through a n-type finFET and a p-type finFET, andwherein at least two fins extend into a source drain region on one sideof said gate stack and forming a carbon-doped silicon layer on said atleast two fins within said source drain region. The method can includegrowing in-situ phosphorous doped silicon on at least one of the atleast two fins.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various embodiments of the invention, in which:

FIG. 1 sets forth steps of a process flow according to the presentinvention. FIG. 1 also presents steps of a current process flow for easeof comparison.

FIGS. 2-4 illustrate forming an initial structure appropriate forprocessing according to the process flow of the present invention.

FIG. 5 illustrates the footprint of the structure of FIG. 4 as a topview at the substrate surface.

FIG. 6 illustrates a CMOS pair of finFETs at an intermediate processingstage according to the present invention.

FIG. 7 illustrates the footprint of the structure of FIG. 6 as a topview at the substrate surface according to embodiments of the presentinvention.

FIGS. 8 and 9 illustrate a CMOS pair of finFETs at further processingsteps according to the present invention.

FIG. 10 illustrates a completed CMOS pair of finFETs according to anembodiment of the present invention.

It is noted that the drawings of the invention are not necessarily toscale. The drawings are intended to depict only typical aspects of theinvention, and therefore should not be considered as limiting the scopeof the invention. In the drawings, like numbering represents likeelements between the drawings.

DETAILED DESCRIPTION

The present disclosure relates to forming complimentary paired finFETsin integrated circuits. As described herein, ‘complimentary pairedfinFETs’ means to a structure that includes two finFETs (fin-type fieldeffect transistors) formed by complimentary metal on semiconductor(CMOS) processing, one being a n-type finFET in which the source drainregions are doped to increase electron mobility and the other being apFET in which the source drain regions are doped to increase holemobility. It will be understood that when an element as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

FinFET performance can be improved by forming the S/D by in-situ dopedepitaxy, but a drawback of conventional approaches to form paired CMOSfinFETS with in-situ doped S/D regions is that the conventional processinvolves two lithography steps which can leave a ridge on the gate stackbetween the nFET/pFET pair. Another drawback is that the gate spacer inthe second to be formed of the finFET pair is wider than that of thefirst to be formed of the finFET pair.

The inventors have conceived of a process flow to form paired CMOSfinFETs, which process flow overcomes the foregoing problems. Theprocess can best be described with reference to the flow path 100(through the even numbered steps) described in FIG. 1 and the structuresdepicted in FIGS. 2-11.

FIG. 2 depicts a starting structure which includes a plurality of finsformed on a substrate 200. The structure of FIG. 2 can be formed as thefirst step 104 of a method embodiment of the present invention.Substrate 200 can be a semiconductor wafer or a part thereof, such asthe substrate of a single integrated circuit chip. Substrate 200 can bea bulk wafer formed of any conventional semiconductor substrate materialincluding but are not limited to silicon, germanium, silicon germanium,silicon carbide, and those consisting essentially of one or more III-Vcompound semiconductors having a composition defined by the formulaAl_(X1)Ga_(X2)IN_(X3)As_(Y1)P_(Y2)N_(Y3)Sb_(Y4), where X1, X2, X3, Y1,Y2, Y3, and Y4 represent relative proportions, each greater than orequal to zero and together summing to 1. Other suitable substratesinclude II-VI compound semiconductors having a compositionZn_(A1)Cd_(A2)Se_(B1)Te_(B2), where A1, A2, B1, and B2 are relativeproportions each greater than or equal to zero and sum to 1.Furthermore, a portion of or the entirety of substrate 200 can bestrained.

Substrate 200 preferably includes an oxide layer (not show) on thesurface 201 between the fins. Optionally substrate 200 is a SOI or ETSOIsubstrate, the fins being formed of the semiconductor layer overlyingthe oxide insulator layer such that the oxide layer also underlies thefins and separates the fins from the bulk of substrate 200. In the caseof substrate 200 being an SOI structure, the fins can be formed byremoving the entire top semiconductor layer between adjacent fins, suchas by spacer imaging transfer. The insulator layer of such SOI structurecan be a conventional oxide material, e.g., silicon dioxide (SO₂), oranother insulating material. The material of the fins can be silicon,silicon germanium, or any suitable semiconductor.

In embodiments, all of fins can be equally spaced, but the invention isnot so limited. A first group of fins 291 can be associated with thenFET region 290 and a second group of fins 251 can be associated withthe pFET region 250. FIG. 2 depicts exactly two fins associated witheach of the nFET and the pFET, but each finFET can have any number offins (including just one fin) as long as the gate traverses at least onefin.

FIG. 3 illustrates a gate stack 210 formed across the fins 291 andcontinuing across the fins 251. Gate stack 210 includes a gatedielectric 212, a gate electrode 214 and a cap layer 216. The structureof FIG. 3 can be formed as step 106 of process flow 100 and can beformed by conventional processing. In embodiments, gate dielectric canbe an insulating material such as silicon oxide, silicon nitride,silicon oxynitride, boron nitride, high-k materials, or any combinationof these materials. Examples of high-k materials include but are notlimited to metal oxides such as hafnium oxide, hafnium silicon oxide,hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide,zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride,tantalum oxide, titanium oxide, barium strontium titanium oxide, bariumtitanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide,and lead scandium tantalum oxide. The high-k may further include dopantssuch as lanthanum, aluminum, gate electrode 214 can be polysilicon oramorphous silicon, germanium, silicon germanium, a metal (e.g.,tungsten, titanium, tantalum, ruthenium, zirconium, cobalt, copper,aluminum, platinum, tin, silver, gold), a conducting metallic compoundmaterial (e.g., tantalum nitride, titanium nitride, tungsten silicide,tungsten nitride, ruthenium oxide, cobalt silicide, nickel silicide),carbon nanotube, conductive carbon, or any suitable combination of thesematerials. The conductive material may further comprise dopants that areincorporated during or after deposition, and cap layer 216 can be anitride layer such as silicon nitride or oxide, oxynitride.

In step 108 a spacer material can be deposited as a conformal layercovering the gate stack 210, covering the fins to the extent they extendinto or through a source drain region on either side of the gate, andalso covering the substrate surface 201. The process flow is describedfor the case of this spacer material being silicon nitride but anymaterial that can form a spacer can be used by appropriate processadjustments such as to the etch chemistries. A directional etch such asreactive ion etch (RIE) applied to the conformal spacer layer can removethe spacer material from all horizontal surfaces and leave a spacer onthe sidewalls of the gate as well as on the sidewalls of the fins. Ifthe gate stack is higher than the fins, then continuing the etch canopen the fins, that is, a ‘pull-down’ etch can remove the spacermaterial from the fin sidewalls while retaining a narrow spacer 220 onthe gate as shown in FIG. 4 and corresponding to step 110 in FIG. 1.

A top view illustrating the footprint of each element of the structureof FIG. 4 at the substrate surface 201 is provided in FIG. 5. Since thenFET spacer and pFET spacer are formed simultaneously, the nFET spacerthickness 229 is the same as the pFET spacer thickness 225. Note thatthis view depicts exposed fin ends 253 and 293, but the fins may extendfurther such that a fin end is not actually exposed.

According to step 112, the next step is to grow a layer of carbon dopedsilicon (Si:C) on the fins of both the nFET (295) and the pFET (255).The inventors have discovered that such Si:C layer can help to balancethe diffusion rates of the different dopants of the FET pair. Morespecifically, it has been observed that carbon doped silicon can reducethe rate or distance of phosphorus diffusion from an ISPD region. Thislayer can therefore enable activating both FET extension regions of aCMOS finFET pair with just a single anneal to drive in dopant.

This Si:C layer can be grown in the same chamber used for other epitaxysteps or in a dedicated Si:C epitaxy chamber. Greater carbon content ora thicker Si:C layer can slow phosphorus diffusion without greatlyaffecting the rate of boron diffusion, so the carbon content andthickness can be tailored as needed. The Si:C layer can be in the rangeof 1 nm to 10 nm thick and can include carbon in the range of 0.2 to3.0%. In a preferred embodiment, the Si:C layer is can be quite thinsuch as in the range of less than 5 nm, or between 1 and 5 nm thick,e.g., 3 nm thick, and can have between 2 and 2.5% carbon. The Si:C layercan be formed by traditional epitaxial processing, or a cyclicalprocess.

One such cyclical process includes a first epitaxial deposition. Onexposed semiconductor surfaces, the growth will be single crystalline.Elsewhere, the growth will be amorphous. An etch using only an etchantgas such as chlorine or HCl or Cl2 can preferentially attack theamorphous material. This etch can be timed to leave some crystallinematerial while removing all the amorphous material. Repeating thissequence can build up a desired thickness of crystalline material.

According to an embodiment, after a sufficient thickness of Si:C isformed, the conditions in the chamber can be adjusted to fill the pFETsource drain (256) with material such as boron doped silicon germaniumthat promotes hole mobility, and simultaneously fill the nFET sourcedrain (257) with the same material. For example if the fin material issilicon, a preferred pFET source drain material can be silicon germaniumhaving germanium content in the range of 5% to 80%, or preferablybetween 20% and 60%. The optimal germanium content of the SiGe sourcedrain can be selected based on design preferences and in embodiments canbe about 40%. In preferred embodiments the source drain filling step 114can utilize epitaxial growth conditions that promote in-situ boron dopedformation of SiGe to merge the source drain regions in both the nFET andpFET regions. The conditions can be tuned to incorporate in the SiGe aboron content in the range of 1×10¹⁹ cm⁻³ to 2×10²¹ cm⁻³, or preferablybetween 2×10^(°)cm⁻³ to 7×10²⁰ cm⁻³.

In embodiments the fins can be oriented such that a 110 crystal surfaceforms the sidewalls 254 and 294 which can promote lateral epitaxialgrowth. Growth conditions can be tuned to permit the lateral growthfaces to meet with minimal vertical growth. FIG. 7 illustrates thevarious layers on the surface 201 under the filled source drain regions.In the case that fins are cut or in any event do not extend completelythrough the source drain region, the Si:C layer will form over the finend as well as over the fin top surface and sidewalls as shown withinfilled source drain 257B. In the case that fins continue to an adjacentgate such that no fin end is exposed within a source drain region, theSi:C layer will form just over the fin top surface and sidewalls asshown within filled source drain regions 257A and 256. The side viewfigures, such as FIGS. 6 and 8, depict this latter case from a verticalplane through line AA.

According to step 116 of process flow 100, the next step is to deposit ahard mask 230. The material of layer 230 can be any material appropriateas a hard mask; in the embodiment illustrated in FIG. 8, layer 230 is athin layer of silicon nitride deposited as a conformal layer. One optionis to deposit layer 230 by in-situ radical assisted deposition (iRAD) asdescribed athttp://www.thefreelibrary.com/Tokyo+Electron+(TEL)+Introduces+Plasma-Enhanced+Batch+Thermal+CVD...-a0133833280.Layer 230 can otherwise be deposited by other known conformal depositionprocesses such as LPCVD (low-pressure chemical vapor deposition), ALD(atomic layer deposition), etc.

A photoresist layer can be patterned (118) to form a soft mask 240covering the pFET region 250 and exposing the nFET region 290. Adirectional etch (120), such as nitride RIE, can remove the exposedhorizontal surfaces of layer 230. At this point, the entire pFET regionis covered by soft mask 240 and hard mask 230. In the nFET, sidewallsalong the sides of the nFET S/D (‘sidewalls’ referring to those surfacesin the nFET region that are generally perpendicular to the direction ofetch 120) are covered by a spacer 239 formed by etch of layer 230, andthe top of the nFET S/D, material 257, is exposed.

Referring now to FIG. 9, the softmask can be removed such as by ashingor wet etching. An etch step 122 can be performed to selectively removematerial 257 from the nFET S/D region. The dummy fill 257 (e.g., SiGe)can be removed using a wet etch process. In some embodiments the wetetch composition includes H₂O₂, NH₄OH and water. In other cases, thedummy fill 257 can be removed in a gas mixture containing HCl. Forexample, an HCl etch can selectively remove ISBD SiGe. The Si:C layer295 protects fins 291 in the nFET S/D region during removal of material257. The nFET S/D 296 can be epitaxially grown in step 124 with anelectron-mobility-enhancing material such as in-situ phosphorus-dopedsilicon (ISPD Si). Alternatively, epitaxial silicon can be grown to fillthe nFET S/D followed by an implant to add dopant such as phosphorus.

As noted above, the spacer 229 in the nFET is the same as the spacer 225in the pFET, resulting in the same offset between the epitaxial S/D andthe channel for both of the pair of CMOS finFETs.

Optionally, spacer 239 can be removed such as by an aqueous solutioncontaining a mix of hydrofluoric acid and Ethylene Glycol. Similarly,the hard mask 230 on the pFET can optionally be removed. If it is notremoved, then the cap layer in the pFET includes 216 and 230, andtherefore would be thicker than the cap layer in the nFET. In any event,since only one hard mask is used according to the process of the presentinvention, there is no residual ridge of hard mask material between theabove described pair of CMOS finFETs.

The extensions (which includes the fin portion under the spacer andextends a certain amount from the edge of the gate towards channel) ofboth FETs can be activated by a single anneal (step 128) Anneal 128 canbe achieved using a rapid thermal anneal tool to perform spike anneal,for example, to 1050C. Other anneal techniques include flash anneal,laser anneal, etc.

This invention is described by reference to examples but the scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal claim language.

We claim:
 1. A structure of a pair of finFETs in an integrated circuit,the structure comprising: a gate stack disposed over a plurality of finson a integrated circuit substrate, a nFET formed on a first portion ofsaid gate stack, said nFET including a first subset of said pluralityand including a nFET source drain region on both sides of said firstportion, wherein at least one fin of said first subset extends laterallyfrom said first portion into said nFET source drain region, a pFETformed adjacent to said nFET on a second portion of said gate stack,said pFET including a second subset of said plurality and including apFET source drain region on both sides of said second portion, whereinat least one fin of said second subset extend laterally from said secondportion into said pFET source drain region, and a carbon doped silicon(Si:C) layer formed on said at least one fin of the pFET and also onsaid at least one fin of the nFET.
 2. The structure of claim 1 furthercomprising a boron doped epitaxial material in said pFET source drainregion.
 3. The structure of claim 1 further comprising a phosphorusdoped epitaxial material in said nFET source drain region.
 4. Thestructure of claim 1 wherein phosphorus doped epitaxial silicon in saidnFET S/D is separated from said at least one fin of the nFET by saidcarbon doped silicon layer.
 5. The structure of claim 1 furthercomprising a gate spacer formed on a sidewall of said gate stack withineach of nFET and pFET, wherein the gate spacer within the nFET is thesame width as the gate spacer within the pFET.
 6. The structure of claim5 wherein said gate spacer in the pFET is the same material as the gatespacer in the nFET.
 7. The structure of claim 1 wherein said carbondoped silicon layer is formed to a thickness between 1 and 5 nm.
 8. Thestructure of claim 1 wherein the carbon content of said carbon dopedsilicon layer is between 2 and 2.5%.
 9. A structure comprising an n-typeand a p-type pair of finFETs in an integrated circuit wherein eachfinFET of said pair includes a gate electrode disposed over a set offins, the structure further comprising: a first gate spacer in saidn-type finFET and a second gate spacer in said p-type finFET, where thethickness at the base of said first gate spacer is the same as thethickness at the base of said second gate spacer.
 10. The structure ofclaim 9 wherein said n-type finFET and said p-type finFET are bothformed from a single gate stack that extends continuously between saidn-type and p-type finFETs, the structure further comprising a hard caplayer disposed directly on said single gate stack wherein a maximumthickness of said hard cap layer at a point between said n-type and saidp-type finFETs is less than or equal to the thickness of said hard caplayer within said p-type finFET.
 11. The structure of claim 9 whereinsaid n-type finFET and said p-type finFET are both formed from a singlegate stack that extends continuously between said n-type and p-typefinFETs, the structure further comprising: a hard cap layer disposeddirectly on said single gate stack wherein said hard cap layer lacks aregion between said n-type and said p-type finFETs having a thicknessgreater than the thickness of said hard cap layer within said p-typefinFET.
 12. The structure of claim 9 further comprising: a Si:C layerformed on at least one of said set of fins in said n-type finFET, and aSi:C layer formed on at least one of said set of fins in said p-typefinFET.
 13. A method to form a complimentary pair of finFETS comprising:on a structure that includes a gate stack disposed over a plurality offins, wherein said gate stack extends through a n-type finFET and ap-type finFET, and wherein at least two fins extend into a source drainregion on one side of said gate stack, forming a spacer on said gatestack, and forming a carbon-doped silicon layer on said at least twofins within said source drain region.
 14. The method of claim 13 furthercomprising: merging said at least two fins within said source drainregion by epitaxially growing ISBD SiGe.
 15. The method of claim 14further comprising: replacing said ISBD SiGe between said at least twofins within said source drain region by epitaxially growing ISPD Si. 16.The method of claim 13 wherein a first of said at least two fins extendsinto a source drain region associated with said n-type finFET and asecond of said at least two fins extends into a source drain regionassociated with said p-type finFET, the method further comprising:growing SiGe from said second of said at least two fins and growingsilicon from said first of said at least two fins.
 17. The method ofclaim 16 wherein said step of growing SiGe is achieved by in-situboron-doped epitaxial growth.
 18. The method of claim 16 wherein saidstep of growing Si is achieved by in-situ phosphorus-doped epitaxialgrowth.
 19. The method of claim 13 further comprising a single annealstep to activate an extension of both said n-type finFET and p-typefinFET.
 20. The method of claim 19 wherein said single anneal step is arapid thermal anneal.